Unfortunately, public domain isn't everything. There are lots of works that we know of and that we should be able to share with one another, but it's hard to come by copies. To give one example:

The novel Red Harvest from 1929 is listed as entering the public domain next month. But the thing is that prior to being published as a novel, it was serialized in Black Mask magazine, and since all installments were published pre-1929, they're all already in the public domain.

The trouble, though, is that despite being public domain, actually getting your hands on these issues, whether in real life or figuratively as scans is something that poses a challenge—we simply don't have easy access to this material.

And that goes for lots of other stuff that we know about but don't have copies at hand.